Conventionally, a flowmeter for measuring a large flow rate (e.g., 5 CCM (mL/min) or more) measures a flow rate by providing, in addition to a sensor pipe, a bypass pipe to branch fluid, because of characteristics of the sensor.
However, in particular, in the case of measuring a flow rate of highly viscous fluid, and in some other cases, air bubbles are retained in inlets of the sensor and the bypass pipes. As a result, there arises a problem that a branch flow ratio between the fluid flowing into the sensor pipe and that flowing into the bypass pipe is changed, which causes a measurement error.
On the other hand, as described in Patent document 1, there is a flowmeter for a small flow rate, which uses a single pipe without branching fluid. This flowmeter is adapted such that a part of a flow tube through which the fluid flows is cooled by an electronic cooling apparatus (Peltier element or the like), and includes: a first temperature detecting section for detecting a temperature of a cooling region of the flow tube; a second temperature detecting section for feedback control and detecting a temperature of a portion of the cooling apparatus, which is away from the flow tube; and a third temperature detecting section for detecting a temperature of a non-cooling region on an upstream side of the cooling region of the flow tube. Also, the flowmeter controls the cooling apparatus such that a difference (T3−T2) between a detected temperature T2 by the second temperature detecting section and a detected temperature T3 by the third temperature detecting section becomes constant. The flowmeter measures a temperature change upon flow of the fluid through the flow tube by the first temperature detecting section, and measures a flow rate of the fluid on the basis of a temperature difference (T1−T2) between the detected temperature T2 by the second temperature detecting section and that T1 by the first temperature detecting section.
However, in the case where this flow meter is applied to a large flow rate measurement, the fluid passes through the flow tube at high speed, so that the fluid flowing into the cooling region is not sufficiently cooled, and therefore a value of the temperature difference (T2−T1) increases (see left-hand side of FIG. 7).
Also, there exists a problem that, as illustrated in FIG. 8, because of the large flow rate, temperature distribution occurs in the electronic cooling apparatus, and therefore even if the control is performed on the basis of the detected temperature T2 by the second temperature detecting section, the large flow rate fluid cannot be sufficiently cooled.
As a result, there arises a problem that a curve indicating ΔTX % of large flow rate/ΔT100% of large flow rate, which represents linearity at a flow rate X % of the large flow rate, is separated from a linear line (linearity is deteriorated) (see right-hand side of FIG. 7). Accordingly, there arises a problem that, for example, a small variation of a zero point of the first temperature detecting section due to external disturbance causes a large measurement error (see right-hand side of FIG. 7).
Also, it is possible to increase a capacity of the cooling apparatus; however, there arises a problem that the entire apparatus is increased in size.    Patent document 1: Japanese Unexamined Patent Publication No. 2004-45290